The invention relates to a fully variable power transmission apparatus in which rotary power applied to an input member is transferred to an output member, with means to permit a continuous or "stepless" change in speed and torque between the input and output members.
Many different types of variable speed power transmissions are available, some types using trains of journalled toothed gears, or chains and sprockets, or pulleys and belts. Many types of transmissions provide a relatively limited speed range or torque ratio, with a number of pre-selected ratios provided between minimum and maximum gear ratios. Such ratios are in so-called "steps" or are discontinuous, i.e. to progress through the full range of gears a series of incremental steps must be taken, intermediate positions between the steps being unattainable. That is, this type of transmission does not provide a continuously variable speed or, torque throughout the entire range between minimum and maximum gear ratios.
While attempts have been made to produce "stepless" or continuously variable transmissions, many additional complications are usually introduced often with limitations on maximum power that can be transmitted. A stepless transmission has the advantage of providing continuous variations in torque and speed between input and output members, but usually the range of gear ratios is relatively limited compared with conventional "stepped" transmissions as previously described.
Examples of stepless gear transmissions are shown in U.S. Pat. No. 4,411,165 issued to Evans in 1983 and in U.S. Pat. No. 4,800,768, issued to Kazuta in 1989. While both of these Patents disclose stepless transmissions which Provide a relatively wide gear ratio range, in the applicant s opinion the designs appear to be limited to relatively low power applications, and it is anticipated that difficulty can be experienced in adapting the patented transmissions to heavy duty application. Both patents disclose an input pinion surrounded by four toothed racks which are connected together as two pairs of oppositely disposed racks, and which transfer power to an output rotor. Spacing between axes of the input pinion and output rotor is variable, and the transmission can be likened to an input pinion driving an output annular gear of variable diameter. The pinion is engaged by the racks in sequence which rotate thereabouts so that there is intermittent meshing between the pinion and the racks in sequence. Both patents disclose resilient means to permit the racks to move radially relative to the pinion.
One problem that appears to be common to both patents is the difficulty of ensuring smooth transfer of torque from the pinion gear to each rack in sequence. As the pinion rotates, each rack is engaged by the pinion in sequence, and thus there is constant engagement and re-engagement of racks by the pinion. Even for a constant speed ratio between the input and output members, difficulty can be experienced when the pinion gradually disengages from one rack, and re-engages with the next rack in sequence. When the gear ratio is actually changing, by changing spacing between axes of the input and output members, the difficulties of ensuring a smooth transfer of torque between the Pinion and the racks is increased. Sometimes a tooth of the pinion can come into direct tip-to-tip contact with an opposing tooth of a rack about to be engaged. If this occurs, the resilience provided in the mounting of the rack Permits the rack to move generally radially outwardly to some extent, but this movement is often insufficient to Prevent high forces from being generated between the pinion and the rack, which can cause damage to the rack. In any event, when such tip-to-tip contact occurs, an impulsive load is inadvertently applied to the rotor, causing intermittent output of power therefrom.
Furthermore, in each of the two said patents, relatively high friction forces can be generated between sliding members, with a corresponding loss of power. Furthermore, the direction of transfer of forces between some adjacent components is less than optimum, significantly increasing mechanical losses in the transmission. In addition, in the patents above, the teeth of the rack opposite to the rack in engagement with the pinion sweep across an outer surface of the cam means. Sweeping gear teeth across a cam surface can produce excessive wear in at least one of the components, which increases undesirable clearance between components, contributing to rough running of the apparatus.